1. Field of the invention
This invention relates to underwater cryogenic pipeline systems. In particular, the invention relates to underwater liquified natural pipeline gas systems that are designed for use in ice infested waters and are adapted to be readily maintained or repaired.
2. Description of related art
The state of the art of underwater liquified natural gas (LNG) pipeline systems is outlined in substantial detail in U.S. Pat. No. 4,718,459, titled "Underwater Cryogenic Pipeline System", filed Feb. 13, 1986 and having Ser. No. 829,054, which is hereby incorporated by reference.
Several substantial design considerations for LNG pipeline systems are discussed in my copending application identified above. They include: material selection for durability and toughness; compensation for contraction of the pipeline; insulation to reduce heat loss; and anchoring. Another consideration in LNG pipeline design is maintenance and repair of the pipeline. Cryogenic pipeline systems, except those using high nickel content steel, and in particular 36% nickel (or "Invar") steel, rely on expansion joints, such as bellows or pipe loops, to compensate for thremal contraction in the system. These expansion joints are where much of the axial movement in the LNG pipeline system occurs. In a metal bellows expansion joint the integrity of the bellows is important because the metal bellows is thinner than the inner pipe wall and is subject to stresses from both pressure loadings and from movement of inner pipe. Thus, it may be desirable to provide some way to access the expansion joints after the pipeline system is installed so that the expansion joints may be inspected or, if necessary, repaired or replaced.
Two approaches to LNG pipeline system maintenance and repair are briefly discussed below. In U.S. Pat. No. 3,379,027 to Mowell, the pipeline may be disconnected at its offshore end and withdrawn from the protective casing if repairs are necessary. In underwater tunnel LNG pipeline systems, the pipelines are in a watertight tunnel that may be entered for repairs if needed. An example of such a system is the Cove Point, Md. LNG receiving terminal underwater tunnel system that is described in several publications.